1. Field of Invention
The present process relates to a process for producing perchloromethyl mercaptan by chlorination of carbon disulfide in the presence of aqueous hydrochloric acid. More particularly, the invention comprises chlorinating carbon disulfide in the presence of 5-38% hydrochloric acid in the presence of high concentrations of sulfuric acid, enabling one to more effectively utilize by-products of the reaction and to thereby increase the economics of prior processes. Specifically, aqueous sulfuric acid replaces water as a feed to the reactor in order to facilitate recovery of hydrochloric and sulfuric acids.
2. Prior Art
The preparation of perchloromethyl mercaptan (PCMM), also known as trichlormethane sulfenyl chloride, by chlorinating carbon disulfide is well known to those skilled in the art. Since first proposed by Rathke [Ber. 3,859 (1870)] much attention has been devoted to the purification and isolation of perchlormethyl mercaptan and to improving yields thereof. Perhaps the most significant problem was to find a method for inexpensively producing a product which is substantially free of sulfur chlorides and in particular sulfur monochloride, which has a boiling point very close to that of perchloromethyl mercaptan. Numerous multistep processes were proposed for removing sulfur chlorides as shown in U.S. Pat. Nos. 2,664,442; 2,666,081 and 2,647,143. A process for simultaneously producing perchloromethyl mercaptan substantially free of sulfur chlorides was described in Masat et al, U.S. Pat. No. 3,544,625 wherein it was disclosed that the sulfur chlorides could be decomposed to hydrochloric and sulfuric acids during chlorination of carbon disulfide if the chlorination were conducted on a mixture of carbon disulfide and an aqueous solution of hydrochloric acid. The only sulfuric acid occurring in the process was that generated by the reaction itself.
While the Masat el al process represents an advantageous method for producing perchloromethyl mercaptan which is substantially free of sulfur chlorides, it has one serious disadvantage. It produces a by-product acid mix containing from about 10-15% sulfuric acid and generally from about 20% to about 35% hydrochloric, balance mostly water. This by-product mix must be disposed of or utilized economically if the process is to be economical as well practicable from an effluent pollution standpoint. As the process was heretofore practiced, the by-product mix is either disposed of as such or neutralized and disposed of. This is wasteful and expensive. It has also been proposed to use the by-product mix as neutralization acid, but this again represents an uneconomical use and one for which there is frequently no demand.
A much more desirable solution would be to conduct the chlorination in a manner which would permit one to recover commercial grade hydrochloric acid from the by-product mix. The recommended means for accomplishing this to date is by utilizing submerged combustion of the by-product mix and this is so expensive in terms of energy requirements that it is for all practical purposes commercially unacceptable.
The difficulty in separating the respective acids from the by-product mix arises by virtue of the fact that hydrochloric acid and water form a maximum boiling azeotrope of about 20 wt. % HCl at atmospheric pressure. Distillation of a hydrochloric acid solution of less than azeotropic concentration produces hydrochloric acid having an acid concentration no greater than the azeotrope concentration. Conversely distillation of a hydrochloric acid solution of greater than azeotropic concentration enables one to remove as commercial grade hydrochloric acid only that amount which is in excess of the azeotropic concentration. The balance stays behind as the azeotrope is distilled off as such.
It has now been found that if chlorination may be conducted in the presence of high concentrations of sulfuric acid, preferably recycled from the process, one may substantially increase the quantity of high quality hydrochloric acid which may be recovered from the aqueous by-product stream by a simple distillation. If the chlorination is conducted in the presence of sufficient sulfuric acid to produce an aqueous by-product phase having a sulfuric acid concentration of 38% or greater, the excess sulfuric acid may be similarly recovered substantially free of hydrochloric acid.
In the following description and claims, all percentages are given on a weight basis unless it is expressly stated otherwise.